Radar tomography for NDT: comparison of techniques

Feasibility and potential of tomography by Ground Penetrating Radar are investigated through experiments on laboratory models. The aim is the development of radar tomography procedures for inspection of structures like walls or pillars in historical buildings. Two different approaches are explored to satisfy high-resolution requirements. The first approach improves the results of classical traveltime (TT) and amplitude tomography (AT) on thin straight or curved rays through a progressive reduction of the null space of the problem. TT is a quantitative tool based on the thin ray assumption that allows a good tradeoff between robustness and resolution. AT is as robust as TT, but its results have only qualitative contents, since the energy transferred to the medium is basically unknown and the scattering effects are not taken into account. In the second approach, GPR is considered as a diffracting source, so that migration (MIG) and diffraction tomography (DT) are applied to overcome the geometrical optic approximations. While DT is in principle the best tool to invert the scattered field and to achieve the maximum resolution, MIG can be a more robust solution that requires less preprocessing of the data. All these advantages and drawbacks of the different approaches are discussed with some examples on synthetic and real data.